The present invention relates to an inverter type variable speed drive intended to monitor/control an electric motor. More specifically, the invention relates to a device for powering a control unit of a variable speed drive and to a method for powering up a variable speed drive.
In a known manner, inverter type variable speed drives include a main rectifier bridge or module, the purpose of which is to rectify a voltage from an external AC electrical power source (for example, a 380 Vac three-phase electrical power network) in order to deliver a DC voltage to a main continuous bus (for example in the order of 400 to 800 Vcc according to the conditions of use of the motor). Variable speed drives hence comprise an inverter module, the purpose of which is, from this main bus, to control a three-phase electric motor with a voltage of variable amplitude and frequency. To this end, the main rectifier module and the inverter module are equipped with power electronic semi-conductor components, for example consisting of one diode and one thyristor per phase for the rectifier module and consisting of two power transistors and two diodes per phase for the inverter module.
An electronic control unit is in particular tasked with controlling the thyristors of the main rectifier module and the transistors of the inverter module. This control unit is usually powered by a switch mode power supply (or SMPS) of the variable speed drive.
The switch mode power supply is generally powered from the main bus voltage. Nevertheless, at the moment the variable drive is powered up, the main bus is not yet charged and therefore does not supply current. Hence the switch mode power supply cannot power the control unit, which is therefore incapable of controlling the thyristors of the main rectifier module, thus preventing the main bus from becoming charged. It is therefore necessary to provide an additional power source for the switch mode power supply when the variable drive is being powered up, before the main bus is charged.
This additional source may come from an auxiliary rectifier module comprising one rectifying diode per phase and connected between the three-phase external source and the switch mode power supply. This additional power source hence supplies an auxiliary continuous bus connected to the switch mode power supply, enabling the latter to power the control unit as soon as the variable drive is powered up so as to be able to control the thyristors of the main rectifier module in order to thus charge the main continuous bus.
However, with this type of power circuit, a loop can then arise between the main bus, the auxiliary bus and a possible earth-return circuit via the external power network, especially in the event of a slight leakage to earth from the variable drive (not detected by a protection circuit) or if long screened motor cables are used (in particular if the motor to be controlled is remote from the variable drive or if the variable drive controls several motors in parallel). In that case, when the main bus voltage is high, for example during motor braking phases, there is then a risk of a very significant overvoltage on the auxiliary bus leading to a danger of destruction or damage of the switch mode power supply and/or of the auxiliary bus capacitance.
To prevent this phenomenon, one solution involves using two separate switch mode power supplies. A first power supply is connected to the main bus and powers a control unit tasked with controlling the power transistors of the inverter module. A second auxiliary power supply is connected to the auxiliary bus and powers a control unit tasked with controlling the power thyristors of the main rectifier module, at least during a startup phase. Nevertheless, this solution turns out to be expensive and cumbersome.
Another solution involves using only rectifier diodes and resistances in the main rectifier module, without needing power thyristors and therefore without needing the control unit for charging the main bus. Nevertheless, this solution can be considered only for small variable drives since it requires the use of high power resistances and a bulky contactor. Furthermore, it does not enable the charging of the main bus to be started up gradually and in a regulated manner.